1. Field of the Invention
The present invention relates to printing and, more particularly, to printing on re-writable media.
2. Description of the Related Art
The majority of printed paper is read once or twice then discarded. Not only is this wasteful of a valuable natural resource (trees), but paper constitutes a significant volume of waste disposal and recycling. There is much interest in providing a paperless office through electronic displays and the Internet. Users, however, find displays to be an inferior alternative to the printed page over a wide range of parameters, not the least of which is eye strain. Thus, there is a growing need and market for a paper or paper-like sheet that can be electronically printed, erased and re-used.
Electrostatically polarized, dichroic particles for displays are well known. Published work by Jacques Pankove of RCA dates back to at least March 1962 (RCA Technical Notes No. 535). Dichroic spheres having black and white hemispheres are reported separately for magnetic polarization by Lawrence Lee, and for electrostatic polarization by Nick Sheridon of Xerox, as early as 1977 (S.I.D. Vol. 18/3 and 4, p. 233 and 239, respectively).
Xerox has been most active in developing dichroic spheres for displays and printer applications. Xerox U.S. Pat. No. 4,126,854, issued Nov. 21, 1978 to Nick Sheridon, describes a dichroic sphere having colored hemispheres of differing Zeta potentials that allow the spheres to rotate in a dielectric fluid under this influence of an addressable electric field. In this, and subsequent U.S. Pat. No. 4,143,103, issued Mar. 6, 1979, Sheridon describes a display system wherein the dichroic spheres are encapsulated in a transparent polymeric material. The material is soaked in a dielectric fluid plasticizer to swell the polymer such that cavities form around each dichroic sphere to allow sphere rotation. The same dichroic fluid establishes the Zeta potential electrostatic polarization of the dichroic sphere. In U.S. Pat. No. 5,389,945, issued Feb. 14, 1995, Sheridon describes a printer that images the polymeric sheet containing the dichroic spheres with a linear electrode array, one electrode for each pixel, and an opposing ground electrode plane.
The dichroic sphere has remained a laboratory curiosity over this period in part because of its high manufacturing cost. The most common reported manufacturing technique involves vapor deposition of black hemispheres on the exposed surface of a monolayer of white microspheres, normally containing titanium dioxide colorants. Methods of producing the microspheres and hemisphere coating are variously described by Lee and Sheridon in the above identified S.I.D. Proceedings. More recently, Xerox has developed techniques for jetting molten drops of black and white polymers together to form solid dichroic spheres when cooled. These methods include circumferentially spinning jets, U.S. Pat. No. 5,344,594, issued Sep. 6, 1994. Unfortunately, the colliding drops produce swirled colorant about the resultant sphere and it is difficult to prevent agglomeration of molten spheres when the concentration of droplets emitted approaches reasonable volumes. None of these techniques lend themselves to bulk, large scale production because they lack a continuous, volume process.
Lee has described microencapsulated dichroic spheres within an outer spherical shell to provide free rotation of the colorants within a solid structure. A thin oil layer separates the dichroic sphere and outer shell. This allows the microspheres to be bound in solid film layers and overcomes the need to swell the medium binder, as proposed by Sheridon. This technique, however, is generally described for magnetic dichroic spheres in the above-referenced S.I.D. Proceedings authored by Lee.
Sheridon describes an electrode array printer for printing re-writable paper in U.S. Pat. No. 5,389,945, issued Feb. 14, 1995. Such a printer relies on an array of independently addressable electrodes, each capable of providing a localized field to the re-writable media to rotate the dichroic spheres within a given pixel area. Although electrode arrays provide the advantage of a potentially compact printer, they are impractical from both a cost and print speed standpoint. Each electrode must have its own high voltage driver to produce voltage swings of 500-600 volts across the relatively low dielectric re-writable paper thickness to rotate the dichroic spheres. Such drivers and their interconnects across an array of electrodes makes electrode arrays costly. The print speed achievable through electrode arrays is also significantly limited because of the short nip time the paper experiences within the writing field. The color rotation speed of dichroic spheres under practical field intensities is in the range of 20 msec or more. At this rate, a 300 dpi resolution printer employing an electrode array would be limited to under one page per minute print speed.
Thus, it can be seen that electrode array printing techniques impose resolution, cost and speed limits upon re-writable media printing devices, and hinder the use of these devices in many applications.
Therefore, there is an unresolved need for a printing technique that can quickly and inexpensively print to re-writable media at high resolution.